1. Field of the Invention
The present invention generally relates to an electronic endoscope apparatus including an electronic scope with a solid-state image sensor. More specifically, the present invention is directed to an overvoltage protection means for such a solid-state image sensor employed in the electronic scope of the electronic endoscope apparatus.
2. Description of the Prior Art
A solid-state image sensor such as a change-coupled device (CCD) has been widely utilized in the electronic endoscope apparatuses. As shown in FIG. 1, such a solid-state image sensor is mounted at a tip (distal end) portion of an electronic scope 1, that will converts an optical signal of an image of an object (not shown in detail) under medical examination, into a corresponding electronic signal of this potential image. In general, a length of the electric scope 1 is amount to approximately 1 meter to 5 meters, and a material of the electronic scope 1 is made of a synthetic resin, which may readily induce static electric (electrostatic) energy. Accordingly, there are many possibilities that the static electric energy can sufficiently give serious damages to the solid-state image sensor, e.g., statically break down this image sensor.
Also, the electronic scope 1 has a connector 12 at the other end thereof. The function of this connector 12 is to disconnect the dirty electronic scope 1 from a main body of the electronic endoscope apparatus 2 for cleaning purposes.
As is known, in the main body of the electronic endoscope apparatus 2, the drive signal for CCD 3 is produced and supplied via the connector 12 of the electronic scope 1 to CCD 3, and furthermore, am image processor 4 is employed to process the electric image signal derived from CCD 3, thereby displaying endoscopic images of the biological body under medical examination on a TV monitor (not shown in detail). This processor 4 further employs digital drivers 4A, 4B, 5A and buffer amplifiers 4C, 4D, 5B for amplifying these signals.
More specifically, the drive signals for CCD 3 are the horizontal transfer clock "H-CLOCK" having a high frequency (e.g., 5 to 9 V at 14 MHz), the vertical transfer clock "V-CLOCK" having a low frequency (e.g., 3-value digital signal with -9 V, O V and 15 V), and DC voltages (e.g., overflow drain voltages). Furthermore, a very small picture signal "OS" is outputted from CCD 3, the frequency of which is the same as that of the horizontal transfer clock "H-CLOCK".
As previously explained, since both the horizontal transfer clock "H-CLOCK" and the picture signal "OS" are of the high frequency signal and furthermore the electronic scope 1 employs such long signal lines "L", the signal levels of these RF signals are lowered due to the transmission loss, resulting in deterioration of endoscopic images. Accordingly, some of the conventional electronic endoscope apparatus additionally employ a repeating circuit 5 so as to compensate for the transmission loss of the high frequency signals.
In the above-described conventional electronic endoscope apparatus with such a solid-state image sensor, there are the following problems.
First, since the electronic scope 1 made of the synthetic resin must be frequently cleaned with a cotton gauze during diagnostic operations, electrostatic energy is easy to be induced on this resin scope 1.
Secondly, since electric insulation of the electronic scope 1 is absolutely necessary for preventing averse influences to a human body (this scope is actually and directly inserted into a body cavity of a human body), such an electrostatic energy may be further induced.
Thirdly, since an electronic endoscope apparatus is operable in conjunction with an electrocautery while electrically connecting an electronic scope therewith, electromagnetic leaks may occur in the conducting wire of the electric scope, so that the electric insulating material must be used to manufacture the electronic scope, which may readily induce electrostatic energy thereon.
Furthermore, when the electronic scope 1 is disconnected from the main body of the electronic endoscope apparatus 2, metal pins of the connector 12 are exposed, as represented in FIG. 2, and may be shortcircuited by another metal portion of the main body, e.g., a metal frame connected to ground. Otherwise, discharge energy caused by shortcircuiting the electrostatic energy via these connector pins may give electrical damage to CCD 3 or the repeating circuit 5.
Also, there is another problem that if such an erroneous operation happens to occur that the electronic scope 1 is connected/disconnected with the main body 2 while electric power is supplied from the main body to the electric scope 1, either CCD 3 or the repeater circuit 5 is electrically destroyed.
The present invention has been made in an attempt to solve the above-described various problems, and therefore has an object to provide an electronic endoscope apparatus equipped with an electronic scope having a solid-state image sensor, capable of preventing electric destruction of a solid-state image sensor caused by overvoltages induced on the electronic scope 1.